This invention relates to a novel planar printed-circuit-board (PCB) transformer structure with effective (EMI) shielding effects.
Planar magnetic components are attractive in portable electronic equipment applications such as the power supplies and distributed power modules for notebook and handheld computers. As the switching frequency of power converter increases, the size of magnetic core can be reduced. When the switching frequency is high enough (e.g. a few Megahertz), the magnetic core can be eliminated. Low-cost coreless PCB transformers for signal and low-power (a few Watts) applications have been proposed by the present inventors in U.S. patent applications Ser. No. 08/018,871 and U.S. Ser. No. 09/316,735 the contents of which are incorporated herein by reference.
It has been shown that the use of colorless PCB transformer in signal and low-power applications does not cause a serious EMC problem. In power transfer applications, however, the PCB transformers have to be shielded to comply with EMC regulations. Investigations of planar transformer shielded with ferrite sheets have been reported and the energy efficiency of a PCB transformer shielded with ferrite sheets can be higher than 90% in Megahertz operating frequency range. However, as will be discussed below, the present invention have found that using only thin ferrite materials for EMI shielding is not effective and the EM fields can penetrate the thin ferrite sheets easily.
FIGS. 1 and 2 show respectively an exploded perspective and cross-sectional view of a PCB transformer shielded with ferrite plates in accordance with the prior art. The dimensions of the PCB transformer under test are detailed in Table I. The primary and secondary windings are printed on the opposite sides of a PCB. The PCB laminate is made of FR4 material. The dielectric breakdown voltage of typical FR4 laminates range from 15 kV to 40 kV. Insulating layers between the copper windings and the ferrite plates should have high thermal conductivity in order to facilitate heat transfer from the transformer windings to the ferrite plates and the ambient. The insulating layer should also be a good electrical insulator to isolate the ferrite plates from the printed transformer windings. A thermally conductive silicon rubber compound coated onto a layer of woven glass fibre, which has breakdown voltage of 4.5 kV and thermal conductivity of 0.79 Wmxe2x88x921Kxe2x88x921, is used to provide high dielectric strength and facilitate heat transfer. The ferrite plates placed on the insulating layers are made of 4F1 material from Philips. The relative permeability, xcexcr, and resistivity, xcfx81, of the 4F1 ferrite material are about 80 and 105xcexa9m, respectively.
According to the present invention there is provided a planar printed circuit board transformer comprising at least one copper sheet for electromagnetic shielding.
Viewed from another aspect of the invention provides a planar printed circuit board transformer comprising,
(a) a printed circuit board,
(b) primary and secondary windings formed by coils deposited on opposed sides of said printed circuit board,
(c) first and second ferrite plates located over said primary and secondary windings respectively, and
(d) first and second copper sheets located over said first and second ferrite plates respectively.